1. Field
The present invention relates to a light guide plate and a surface light source device.
2. Related Art
In recent years, as the thickness of a mobile device decreases, a surface light source device which is assembled in a mobile device is also requested to become thinner and thinner. To reduce the thickness of the surface light source device, the thickness of the light guide plate has to be decreased. However, even when the thickness of a light guide plate having a flat plate shape is reduced, it is difficult to reduce the height of an LED light source. In the case of using a thin light guide plate having a flat plate shape, the height of the light source becomes larger than the thickness of an end face (light incident end face) of the light guide plate. As a result, a light source disposed so as to be opposed to the light incident end face of the light guide plate is protruded above the top face of the light guide plate. In the case where the light source is protruded above the top face of the light guide plate, all of light emitted from the light source does not enter the light incident end face of the light guide plate but a part of the light leaks to the outside of the light guide plate, so that the light use efficiency deteriorates.
An example of a light guide plate is given as follows. The light guide plate has a light introduction part thicker than a light guide plate body at an end of the light guide plate body having a flat plate shape. In the top face of the light introduction part, an inclined face which is inclined from a part where the thickness is largest in the light introduction part toward the end of the light guide plate body is formed. Since the thickness of the end face (light incident end face) of the light introduction part is equal to or larger than the height of a light source, the light guide plate can efficiently take the light of the light source into the light introduction part and guide the light to the thinner light guide plate body.
The light which enters the light introduction part from the light incident end face is transmitted to the light guide plate body while being totally reflected between the inclined face and the lower face of the light introduction part. The incidence angle of the light propagating in the light guide plate body when the light is incident on the upper face of the light guide plate body becomes smaller each time the light is totally reflected by the inclined face. Due to this, the light easily leaks from the top face of the light guide plate body positioned in the proximity of the inclined face to the outside of the light guide plate body. In particular, when the inclination angle of the inclined face increases, the leakage of light from the top face of the light guide plate body in the proximity of the inclined face becomes conspicuous.
To reduce the light leakage, a surface light source device as illustrated in FIG. 1A is used. A surface light source device 11 illustrated in FIG. 1A is made of a light source 12 and a light guide plate 13. The light guide plate 13 is obtained by integrally forming a light introduction part 14 having a wedge shape and a light guide plate body 15 having a flat plate shape. The top face of the light introduction part 14 is an inclined face 16 which is inclined from the top face of the upper end of the light introduction part 14 to the end of the light guide plate body 15. The inclined face 16 of the light introduction part 14 has a directivity conversion pattern 17 in which a plurality of V grooves 18 are arranged in the width direction of the light guide plate 13. The top face (light emitting surface 20) of the light guide plate body 15 has a lenticular lens 21. The light source 12 is opposed to the end face (light incident end face 19) of the light introduction part 14.
Light emitted from the light source 12 enters from the light incident end face 19 to the inside of the light introduction part 14 and is guided in the light introduction part 14. Since the light incident on the inclined face 16 in the light introduction part 14 is reflected by the V groove 18 and travels to the lateral direction (width direction of the light introduction part 14), the light incidence angle when the light is incident on the light guide plate body 15 in the proximity of the inclined face 16 becomes large. As a result, light leakage from the top face of the light guide plate body 15 in the proximity of the inclined face 16 is suppressed.
The surface light source device 11 is illustrated in FIG. 1B. Light L introduced in the light introduction part 14 is reflected in the lateral direction each time the light L is incident on the V grooves 18, a part of the light L is gradually guided to a side face of the light guide plate 13. The light L reached the side face of the light guide plate 13 leaks to the outside from the side face of the light guide plate 13 without reaching the light emitting surface 20. The phenomenon causes a light amount loss in the surface light source device 11 and deteriorates the light use efficiency of the surface light source device 11.
There is a case that a plurality of the surface light source devices 11 illustrated in FIG. 1A are arranged continuously along the width direction. In other words, it is a case where a plurality of light sources are arranged at a predetermined pitch so as to be opposed to the light incident end faces in a light guide plate wider than the light guide plate 13 illustrated in FIG. 1A. In this case, light L leaks from a border face of the light guide plates 13 (the position corresponding to the side face of the light guide plate 13 in FIG. 1A) to the adjacent light guide plate 13, and brightness unevenness may occur in both of the light guide plates 13.
A surface light source device 31 is illustrated in FIG. 2. In the surface light source device 31, both side faces of the light guide plate 13 are obliquely cut in the proximity of the light incident end face 19. The obliquely cut faces serve as light reflection walls 32. The light guide plate of this type is described in, for example, JP 2008-16432 Å.
In the surface light source device 31, as illustrated in FIG. 3A, the light L which reaches the side face of the light guide plate 13 while being repeatedly reflected by the upper face (V grooves 18) of the light introduction part 14 and the lower face is totally reflected by the light reflection wall 32 and returns to the center part of the light guide plate 13. As a result, light leakage in the side faces of the light guide plate 13 is reduced, and light use efficiency improves. Also in the case where a plurality of surface light source devices 31 are disposed continuously in the width direction, leakage of the light L to the adjacent light guide plate 13 is suppressed, and occurrence of brightness unevenness is suppressed.
In the surface light source device 31 of FIG. 2, light incident on the light reflection wall 32 has to be totally reflected as many as possible by the light reflection wall 32. Consequently, the light reflection wall 32 has optimum length and optimum inclination angle (angle formed by the light reflection wall 32 and a direction perpendicular to the light incident end face 19 when viewed from a direction perpendicular to the light emitting surface 20), and the adjustment range of the inclination angle of the light reflection wall 32 is narrow. Since the inclination adjustment range of the light reflection wall 32 is narrow, when a width W of the light guide plate 13 decreases, a width D of the light incident end face 19 also decreases as illustrated in FIG. 3B. As a result, there is the case where the width D of the light incident end face 19 becomes smaller than a width “d” of (the light exit window of) the light source 12. In the case where the surface light source devices 31 are continuously disposed along the width direction, when the arrangement pitch of the light sources 12 becomes narrow, the width W of the light guide plate 13 corresponding to one light source is accordingly reduced. As a result, there is the case where the width D of the light incident end face 19 becomes smaller than the width “d” of the light source 12. When the width D of the light incident end face 19 becomes smaller than the width “d” of the light source 12, a part of light emitted from the light source 12 goes out from both sides of the light incident end face 19 and does not enter the light guide plate 13, causing light use efficiency to deteriorate.